Error-correcting laptop memory
(I've marked this "advertising and marketing", too, because I realise I'm probably with a very small minority of users who care about this now. If however Dell would give it the right spin, it could - at least for a while - be a unique selling point: the only really serious mobile workstation in the world...?)
For me, only one thing is really missing to make something like an E6420 laptop my everyday workstation: ECC RAM. That's because somehow, I move enough data around that I actually come across a flipped bit every few weeks or so.
PS: I believe pretty much all of AMD's 64-bit chips have ECC-capable memory controllers, and I suspect that Intel would also be willing to unlock theirs for a big customer like Dell (they're just slicing up the market, see http://www.supermicro.com/support/faqs/faq.cfm?faq=10712 - so please persuade them to make you a "mobile Xeon").